Since the advent of molecular-targeted drugs, it is thought that, in order to maximize the effects of the drugs including antibody drugs, utilization of information for selection of the subjects to which the drugs are to be administered or for optimization of their doses, is effective. As a famous example, it is known that expression of the her2/neu molecule may be an aid for selection of breast cancer patients to whom Herceptin (anti-her2/neu antibody drug) is to be administered (Non-patent Document 1), and thus, it is instructed in the FDA's approval that immunohistological testing of the expression level of the her2/neu molecule in pathological diagnosis of a cancer tissue is indispensable for prescription of the drug. A number of similar cases have been reported, and such drugs are increasing year by year. Such indices have come to be called companion diagnosis (biomarkers in a broad sense). Further, it is known that a therapeutic agent for metastatic melanoma (vemurafenib) is highly effective in patients who have a V600E mutation in the BRAF gene (Non-patent Document 2), and thus administration of the agent to patients having this mutation has been recommended in recent years. Furthermore, when an ALK inhibitor (crizotinib, ceritinib) is prescribed to patients with non-small cell lung cancer in whom the kinase is constantly activated as a consequence of fusion between ALK gene and another gene due to translocation of the ALK gene, the presence or absence of the translocation of the ALK gene is determined beforehand (Non-patent Documents 3 and 4). This also corresponds to the companion diagnosis. Unfortunately, it is becoming clear that the ALK fusion gene test has a problem in that, since the position of the cleavage-fusion slightly varies, its detection by RT-PCR with a single primer setting is not always successful.
Approaches supported by the above concept have been taken in prescription of antibody drugs as companion diagnostic drugs, and the authority has approved such approaches. There are the following combinations: Herceptin-her2/neu expression, cetuximab (anti-EGFR)-EGFR expression, and Poteligeo (anti-CCR4)-CCR4 expression. However, in cases of prescription of cetuximab, the expression level of EGFR does not actually reflect the effect, and it has been revealed that a mutation of the K-ras gene present downstream of the EGFR signal actually reflects the effect (Non-patent Document 5). Also in treatment with Herceptin, nearly half of patients who were prescribed the drug showed recurrence/drug resistance (Non-patent Document 6), and therefore improvement is required from the viewpoint of whether or not the expression levels of the target molecules of molecular-targeted drugs can simply be indicators for maximizing the advantages in the patients.
For example, in recent years, antibody drugs that exert their effects especially in the immune system have been shown to be effective in treatment of tumors and cancers in terms of the performance (in particular, the life-prolonging effect) in clinical trials, and the governmental authority gave manufacturing approval to them. Among these drugs, for example, when an anti-CTLA-4 antibody (ipilimumab) was approved by FDA, there was no major discussion on biomarkers. In contrast, for prescription of an anti-PD-1 antibody (nivolumab; Opdivo) approved by the Ministry of Health and Welfare in July 2014, histological staining has been carried out in clinical trials, based on the scenario that testing of the degree of expression of the PD-L1 molecule, which is a ligand of the PD-1 receptor molecule, in cancer patient tissues may be effective. A plurality of companies are working on development of the above-mentioned antibody drugs and companion diagnosis therefor. In this process, problems have become clear. An anti-PD-L1 antibody is used in tissue staining. Studies are carried out using different anti-PD-L1 antibodies, but there is a variation in the thus evaluated effectiveness possibly due to differences in the reactivity among the antibodies. Several problems, such as heterogeneity caused by the fact that the samples are tissues, and possible false-negative results in relation to sampling, have been pointed out. In view of the above, a blood test method with which homogeneity of samples is more likely to be secured is demanded, but there have actually been no biomarkers that were shown to be satisfactory.
Unfortunately, the above-described companion diagnosis (biomarkers) merely provides information on selection of the subjects to be treated, and still cannot be indices for monitoring of whether or not drugs are producing their effects. Moreover, the indices that are most preferred at clinical sites are indices which allow discussion based on blood tests rather than tissue-level indices. A variety of the so-called biomarkers have been reported, but biomarkers primarily demanded at clinical sites still have not been provided so far.
Since Poteligeo (anti-CCR4 antibody) is prescribed for T-cell type blood cancer, the sample to be tested in this case is a blood cell sample, which is relatively homogeneous. However, in solid cancers, the most common method is tissue staining. In recent years, based on the information that the extracellular domain (Extra Cellular Domain; ECD) of the her2/neu molecule is cleaved and released into blood flow, investigation of the level/degree of expression of her2/neu by a blood test targeting ECD has become possible (Non-patent Documents 7 to 10) as an alternative to the test by tissue staining of the molecule immobilized on the cell surface. However, this still remains to be improved.
The past carcinostatic drugs, including not only traditional carcinostatic drugs such as DNA synthesis inhibitors and protein synthesis inhibitors, but also molecular-targeted drugs that have been newly developed recently, directly target molecules such as membrane antigens and enzymes expressed in tumor cells. In contrast, antibody drugs utilized in novel tumor immunotherapies are attracting attention in recent years. Most of the immune checkpoint antibodies such as anti-PD-1 antibodies discussed therefor do not target molecules expressed on tumor cells, but target molecules expressed on immunocompetent cells. Thus, it can be said that the search for biomarkers is now more difficult. The background which makes the situation even more complicated is that, similarly to an anti-PD-1 antibody, an anti-PD-L1 antibody also exhibits a tumor-growth inhibitory effect. That is, it is primarily reasonable to investigate the expression level of the PD-1 molecule in the immunocyte system for prescription of the anti-PD-L1 antibody. When an anti-PD-1 antibody and an anti-PD-L1 antibody become commercially available at the same time, what indices can really be appropriate biomarkers?
There is a group who studied the immune surveillance mechanism against tumors in order to investigate whether the innate immune mechanism in the human body is capable of recognizing and distinguishing tumors, which are cells generated through alteration of autologous cells. The group reported, as a result, that tumors express tumor antigens with which tumor cells can be distinguished from normal cells, and that those antigens are molecules targeted by lymphocytes. For example, it is reported that tumor exome analysis, which is a special technique using next-generation sequencing, was carried out to reveal that the molecule called spectrin-β2 is targeted by CD8+ T cells (Non-patent Document 11). Such a molecule can also be a biomarker. Similarly, by exome analysis of a tumor tissue from a patient in whom an anti-CTLA-4 antibody was effective, a gene mutation targeted by CD8+ T cells was found (Non-patent Document 12). Thus, such a mutant gene product can also be a biomarker. It may also be possible to infer that, besides biomarkers recognized by CD8+ T cells, which are deduced based on information from the tumor side, molecules responsible for functional regulation of CD8+ T cells can also be biomarkers. That is, functional regulatory molecules such as those expressed in T cells having CTL activity are also likely to be biomarkers. However, the technology gap is still too large to apply such a next-generation sequencing technique to general clinical diagnosis.
It is proposed, as described above, that one possible cause of the immunocompromised state of cancer-bearing patients is the presence of a group of cells expressing immune checkpoint molecules. Recent interest has focused on methods in which functions of such a group of cells are suppressed or eliminated to create conditions where cells and molecules having killer activity against tumors can function, thereby controlling cancers and tumors. It can also be said that the search for biomarkers is even more difficult because of the complicated mechanism. Taking into account the fact that search for biomarkers that reflect therapeutic effects is demanded rather than biomarkers to be used merely as criteria for determining whether drugs are to be prescribed or not, there is an increased difficulty.